[clang] Fix serialized diagnostics edge-cases

The Clang frontend sometimes fails on the following assertion when launched with `-serialize-diagnostic-file <x>`:

```
Assertion failed: (BlockScope.empty() && CurAbbrevs.empty() && "Block imbalance"), function ~BitstreamWriter, file BitstreamWriter.h, line 125.
```

This was first noticed when passing an unknown command-line argument to `-cc1`.

It turns out the `DiagnosticConsumer::finish()` function should be called as soon as processing of all source files ends, but there are some code paths where that doesn't happen:

1. when command line parsing fails in `cc1_main()`,
2. when `!Act.PrepareToExecute(*this)` or `!createTarget()` evaluate to `true` in `CompilerInstance::ExecuteAction` and the function returns early.

This patch ensures `finish()` is called in all those code paths.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D118150